Portable electronic devices are used for many applications. Examples include telephones, personal digital assistants (PDAs), pagers, global positioning systems, digital cameras, and the like. Many of these devices rely on a local power supply with a very limited lifetime. Moreover, many of these devices include a display used to present text, graphics, and images to users.
Displays in common use for mobile devices are primarily based on liquid crystal displays (LCD). Reflective LCD displays take very little power to operate but cannot be seen in the dark, i.e. they require external illumination. Transmissive LCD displays utilize a back-light to provide illumination that is blocked (or not) by pixel elements in a display. The back-light illumination is used regardless of the display content. For example, displaying a black screen requires the same amount of power as displaying a white or colored screen. In contrast, emissive displays, such as organic light emitting diode (OLED) displays, only use power when emitting light so that displaying a black screen requires no power while displaying a white or colored screen does require power.
Most information presented on displays is formatted by a mark-up language compatible with the standard generalized markup language (SGML) specification. Such a language typically specifies the size, font, background, position, etc., of text as well as the location and size of graphic or image elements in the information. A mark-up language provides instructions to a computer controlling the display on how to format the information. For example, the hypertext markup language (HTML) is used for presenting information on Internet web sites.
Information displays with their own illumination source often use a significant fraction of the available power for portable computing and/or communications devices. For example, PDAs and cell phones incorporate a display used for presenting both text and images. The display can be a significant drain on the power supply of the device and the power supply must be recharged at frequent intervals, limiting the available time that the device is usable between charges. This is inconvenient and reduces the usefulness of the device.
There is a need therefore for an improved method for reducing the power used by the display in a portable electronic device.